Set the holders to null after we've reclaimed the sounds.

git-svn-id: svn+ssh://src.earth.threerings.net/nenya/trunk@107 ed5b42cb-e716-0410-a449-f6a68f950b19

src/java/com/threerings/openal/SoundGroup.java

@@ -84,6 +84,7 @@ public class SoundGroup
                 if (source.holder != null) {
                     source.holder.stop();
                     source.holder.reclaim();
+                    source.holder = null;
                 }
             }
         }